Effects of solvent composition on agglomerate structure in catalyst ink for polymer electrolyte fuel cells

The solvent composition of the catalyst ink profoundly impacts both the porous structure and the efficiency of the mass-production process of the catalyst layer for polymer electrolyte fuel cells. However, the underlying mechanism remains far from elucidated. Here, we present the size of aggregate and the fractal dimension of the agglomerate structure as determined by ultra-small X-ray scattering (USAXS) of the catalyst inks with varied solvent composition. By examining the correlations between the structural properties provided by USAXS, rheological properties, and the quantity of ionomer adsorbed on the Pt/C, we elucidate the solvent's influence on the agglomerate structure. In highly polar solvents, akin to water, a considerable number of amphiphilic ionomers adsorb to the Pt/C particles, resulting in the formation of dispersed Pt/C aggregate particles approximately 200 nm in size that cannot be subdivided further into smaller particles even with the use of a homogenizer. As the polarity diminishes with the addition of alcohol content, ionomers desorb, and the affinity between the aggregates and the solvent decreases, leading to the evolution of agglomerates. The observed solvent effect can be rationally characterized by one of Hansen's solubility parameters, which specifies the polar interaction.